Chen K C, Chen J S, Johnson J L
J Bacteriol. 1986 Apr;166(1):162-72. doi: 10.1128/jb.166.1.162-172.1986.
The structural gene (nifH1) encoding the nitrogenase iron protein of Clostridium pasteurianum has been cloned and sequenced. It is located on a 4-kilobase EcoRI fragment (cloned into pBR325) that also contains a portion of nifD and another nifH-like sequence (nifH2). C. pasteurianum nifH1 encodes a polypeptide (273 amino acids) identical to that of the isolated iron protein, indicating that the smaller size of the C. pasteurianum iron protein does not result from posttranslational processing. The 5' flanking region of nifH1 or nifH2 does not contain the nif promoter sequences found in several gram-negative bacteria. Instead, a sequence resembling the Escherichia coli consensus promoter (TTGACA-N17-TATAAT) is present before C. pasteurianum nifH2, and a TATAAT sequence is present before C pasteurianum nifH1. Codon usage in nifH1, nifH2, and nifD (partial) is very biased. A preference for A or U in the third position of the codons is seen. nifH2 could encode a protein of 272 amino acid residues, which differs from the iron protein (nifH1 product) in 23 amino acid residues (8%). Another nifH-like sequence (nifH3) is located on a nonadjacent EcoRI fragment and has been partially sequenced. C. pasteurianum nifH2 and nifH3 may encode proteins having several amino acids that are conserved in other proteins but not in C. pasteurianum iron protein, suggesting a possible role for the multiple nifH-like sequences of C. pasteurianum in the evolution of nifH. Among the nine sequenced iron proteins, only the C. pasteurianum protein lacks a conserved lysine residue which is near the extended C terminus of the other iron proteins. The absence of this positive charge in the C. pasteurianum iron protein might affect the cross-reactivity of the protein in heterologous systems.
巴氏梭菌固氮酶铁蛋白的结构基因（nifH1）已被克隆并测序。它位于一个4千碱基的EcoRI片段上（克隆到pBR325中），该片段还包含一部分nifD和另一个类nifH序列（nifH2）。巴氏梭菌nifH1编码一种与分离出的铁蛋白相同的多肽（273个氨基酸），这表明巴氏梭菌铁蛋白较小的尺寸并非由翻译后加工导致。nifH1或nifH2的5'侧翼区域不包含在几种革兰氏阴性细菌中发现的nif启动子序列。相反，在巴氏梭菌nifH2之前存在一个类似于大肠杆菌共有启动子（TTGACA - N17 - TATAAT）的序列，并且在巴氏梭菌nifH1之前存在一个TATAAT序列。nifH1、nifH2和nifD（部分）中的密码子使用非常有偏向性。在密码子的第三位可见对A或U的偏好。nifH2可以编码一个272个氨基酸残基的蛋白质，它与铁蛋白（nifH1产物）在23个氨基酸残基上不同（8%）。另一个类nifH序列（nifH3）位于一个不相邻的EcoRI片段上，并且已被部分测序。巴氏梭菌nifH2和nifH3可能编码具有在其他蛋白质中保守但在巴氏梭菌铁蛋白中不保守的几个氨基酸的蛋白质，这表明巴氏梭菌类nifH样序列在nifH进化中可能具有作用。在已测序的九种铁蛋白中，只有巴氏梭菌的蛋白质缺乏一个保守的赖氨酸残基，该残基位于其他铁蛋白延伸的C末端附近。巴氏梭菌铁蛋白中这种正电荷的缺失可能会影响该蛋白在异源系统中的交叉反应性。
